Resolve unnecessary_to_owned clippy lint in test suite

    error: unnecessary use of `to_vec`
       --> test_suite/tests/test_de.rs:251:12
        |
    251 |     .chain(ignorable_tokens.to_vec().into_iter())
        |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `ignorable_tokens.iter().copied()`
        |
        = note: `-D clippy::unnecessary-to-owned` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned

test_suite/tests/test_de.rs

@@ -248,7 +248,7 @@ fn assert_de_tokens_ignore(ignorable_tokens: &[Token]) {
         Token::Str("ignored"),
     ]
     .into_iter()
-    .chain(ignorable_tokens.to_vec().into_iter())
+    .chain(ignorable_tokens.iter().copied())
     .chain(vec![Token::MapEnd].into_iter())
     .collect();